You'd actually get less voltage

The way transformers work is they have a primary and secondary coil. Ever see a tesla coil? In a tesla coil there is a bottom coil that is maybe 10-20 turns and the secondary coil (the big mast part) that has thousands if not millions of turns. If the ratio of turns is 1:10 you will (in theory, neglecting losses) have a 10x voltage increase. That is 120v input would equate to 1200v output, and so on. This allows the really high 90-120amps on a standard 20amp 120v circuit. because 120amps at 1200v is really 10amps (again neglecting the losses from the magnetic transformer) on the 120v circuit.

Now if you reversed it 120v into the secondary coil you now have a 10:1 ratio, and you would inefficiently step 120v down to 12v.... so you could start your car or something.

If anyone notices any errors please correct me. It's been a long day

-Max